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COOLED HYDROGEN OR NEON USED AS TRANSFORMER DIELECTRIC Filed June 29, 1964 e Sheets-Sheet e United States Patent 3,396,355 COOLED HYDROGEN 0R NEON USED AS TRANSFORMER DIELECTRIC Bernard Hochart, Rueil-Malmaison, and Pierre Burnier,

Versailles, France, assignors to Societe Generale de Constructions Electriques et Mecaniques (Alsthom), Paris, France, a corporation of France Filed June 29, 1964, Ser. No. 378,529 Claims priority, application France, June 28, 1963,

2,427, Patent 1,368,938 9 Claims. (Cl. 33658) ABSTRACT OF THE DISCLOSURE An electrical inductive apparatus such as a transformer has conductors of very pure metal kept at low temperature, surrounded by a dielectric which is fluid hydrogen or neon at 15 K. to 60 K. The metal may be Cu, Al, Fe, Au, or In containing less than 500 p.p.m. impurities.

Progress in the technology of electrical inductive apparatus such as transformers has been shown in a simultaneous increase in the specific rating and the efficiency of these apparatus. In the course of the last few years remarkable results have been obtained in this field due to the production of ever increasing unitary powers and to the use of improved arrangements and materials.

A limit has, however, now been reached since the increase in the specific rating leads to an increase in stress and strain on the materials (magnetic induction, current density, voltage gradients), and thus to an increase in specific losses, the underlying reasons for which can be traced back to the materials used. Consequently, in spite of progress made in means for the reduction of losses, the general tendency is for the operational temperature to increase.

Generally speaking the properties of the materials be come less and less favourable as their temperature increases. For example an increase in temperature brings about a decrease in the electrical conductivity of conductors, in the saturation induction of magnetic materials, in the burn-out gradient of insulators and increased dielectric losses in the insulators. Consequently, the increase in the specific rating of transformers raises ever graver problems and will soon bring with it a decrease in their efficiency if the use of present techniques is continued.

Attempts have recently been made to combat this tendency to increase operational temperatures, by exploiting the superconductivity of certain metals or alloys. Unfortunately superconductive materials are extremely expensive and require very low temperatures, in practice, that of liquid helium. In order to avoid heat losses a refrigerator has to be used, but the output of these machines is very low at these very low temperatures. Soft superconductive materials, or superconductors are similarly unsuited to the construction of transformers, since their property of superconductivity disappears with a critical value of the magnetic field, which is much lower than that of the fields used in transformers. In order to increase this critical value, hard superconductors have been developed but the superconductive property of these materials is only obtainable over a small section of the conductor and the losses caused in the nonsuperconductive part by the passage of alternating current in the superconductive part are such that it is not economically possible to reduce them.

The present invention has for an object an electrical inductive apparatus which allows the limitations met with so far in joint research into specific rating and increased cfliciency to be obviated, avoided, or at any rate, minimised. A transformer according to the invention is char- 3,396,355 Patented Aug. 6, 1968 ice acterised by the combined use of hydrogen or of neon at a temperature of between 15 K. and 60 K. as a dielectric fluid, and of metals of extreme purity as conductors.

The importance of this novel transformer results from the studies effected on the one hand, on the optimum conditions of use of very pure metals .and on the other hand, on the characteristics of cryogenic fluids, and also on the behaviour of the materials used in the construction of transformers at low temperatures.

Firstly, the optimum conditions of use of very pure metals have been ascertained from studies carried out on the evolution and function of the temperature of the resistivity of these metals and of the output (efficiency) of cooling devices.

In fact, on the one hand, the conductivity of the very pure metals is multiplied by a factor of between and 10,000 where the temperature reaches some 10 K., on the other hand, the electrical losses resulting from these metals must, however slight, be dissipated towards the ambient atmosphere by means of refrigerating or cooling devices, the efiiciency of which is very low as the temperature approaches 0 K.

Curves have been drawn giving the resistivity of metals of varying degrees of purity up to a temperature of several degrees Kelvin. In the same way the efficiency of the refrigerating machines is characterised by the power required to dissipate, towards the ambient atmosphere, unit power taken at the temperature T. This power is assessed by the product of the factor resulting from the Carnot theory (T being the temperature in degrees Kelvin) by a factor M characterising the imperfection of the refrigerating device with respect to the ideal Carnot cycle. It has been found that the factor M could be represented by the following formula drawn up with experimental values measured on mean or low power refrigerating machines:

between 0 K. and 100 K.: M=22 4 (0.1+ between 100 K. and 293 K.: M=4.27--0.775Tl0 This factor M may decrease on refrigerating devices of greater power, particularly at low temperatures.

On the basis of the resistivity curves and the values of between 100 K. and 293 K.: M=4.270.775Tl0 the refrigerating power, the total power T has been calculated which is lost either by the Joule effect or by the power consumption of the refrigerator, in order to maintain at T K. a conductor of a given geometrical form constituted of a given metal, when said conductor is traversed by a given current I:

L representing the length of the conductor and S its section.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which show some embodiments of the invention by way of example, together with some explanatory curves, and in which:

FIGURE 1 shows some explanatory curves,

FIGURE 2 shows a view of the assembly of a transformer according to the invention,

FIGURE 3 shows in section a leg of the transformer of FIGURE 1,

FIGURE 4 shows a section along the line IV-1V of the tank of the transformer according to FIGURE 3,

FIGURE 5 shows a view of an assembly of another type of transformer according to the present invention, and

FIGURE 6 shows in section another type of transformer according to the present invention.

Referring now to the drawings, FIGURE 1 shows several curves given by way of example showing on the ordinate the variation of the power P referred to above, as a function of temperature drawn on the abscissa, the power being expressed as a percentage of that consumed in a copper conductor of the same shape assumed to be at 70 C. (343 K.) and traversed by the same current. The curves A, B, C and D respectively relate to copper, industrial aluminium, very pure aluminium, and iron.

Since the importance of the application of very pure metals relates to obtaining as low a percentage as possible and which is always lower than 100% it is clear from FIGURE 1 that such values of power may be reached in the temperature zone of 15 Kelvin to 60 Kelvin, with certain metals and on condition that said metals have a sufiicient degree of purity.

It has been found for example, that copper, aluminium, iron, and tin, as well as other metals such as silver, gold, indium, etc. may be used provided that their impurity rating is lower than 500 parts per million. This is the significance of the expression very pure used herein to define the metals employed. If possible the impurity rate should be 50 parts per million. For certain of these metals it is possible to obtain such a low impurity rating in an economical manner by very inexpensive metallurgical methods such as electrolysis.

Further studies have shown that the conditions of use a s encountered in a transformer do not greatly affect the conditions of this simplified calculation. Thus the effects of magnetic resistance caused by the helical movement of the electrons in a conductor subjected to a magnetic field does not greatly affect the resistivity value. Specialised studies on this point have shown for example that the increase in resistivity A due to the magnetic resistance could be expressed in the case of aluminium by the following equations:

p being in ohms-centimetres and expressing the iresistivity of the metal under consideration at the temperature T K., H being the intensity of the magnetic field in gauss and A a coefiicient depending upon the purity of the aluminium and equal to unity for example, the resistivity ratio of which between 293 K. and 4.2 K. is 1,350 and 1.3 for example, the ratio of which is 2,600. This equation does not correspond to the generally known theoretical formula:

AH I +BH but perfectly expresses the experimental results.

Further studies have made it possible to show that in order to avoid too great losses arising from eddy currents, very thin conductors are advantageously used, or conductors made from a stack of very thin conductors or by stranding of very fine wires. In order to avoid, in these small-size conductors, a substantial increase in their resistance known as the dimensional effect, and explained by the interaction of the electrons and the walls of the conductors, it has been found that thicknesses or diameters of some 10th or 100th of microns were satisfactory. Other parasitic effects of conduction, such as the dimensional magneto resistive effect and the anomaly of the skin effect, have been found negligible in the conditions of use under consideration.

Secondly, with regard to the study of the characteristics of cryogenic fluids, it has been found that the use of liquid or gaseous hydrogen or neon as dielectric media and coolants in the new transformer was particularly suitable. It has been found, for example, that their dielectric strength at temperatures of between 15 K. and 60 K. and under pressure equal to or greater than atmospheric pressure (or slightly less than atmospheric pressure in the case of liquid), was of the same order as that of the mineral dielectric oils at present used in transformers. For example liquid hydrogen under normal pressure has a burn-out gradient of 680 kv./cm. for an electrode spacing of 0.25 mm. The dielectric losses of these fluids are very slight and are practically impossible to measure. Their very slight viscosity and the high coefficient of heat exchange which they make it possible to reach, render them extremely suitable for the cooling of the coils of the new.transformer. It is thus possible to dissipate one watt per square cm. of coil surface with liquid hydrogen or neon. The viscosity of the liquid hydrogen at 20 K. is 140 micro-poises and that of gaseous hydrogen at 20 K. is 2 micro-poises.

Thirdly, as regards the behaviour of the materials used in the construction of transformers when immersed in hydrogen or neon at temperatures of between 15 K. and 60 K., studies carried out show that their properties are generally more favourable than at ambient temperatures and are thus appreciably more suitable than at the operational temperatures of the transformers at present in use. It has thus been found that dielectric losses are very slight under these conditions, even for very polar or slightly insulated materials at normal temperatures (consequently of reduced quality and price). In the same way the dielectric strength of insulators is greater than that measured at temperatures greater than or equal to ambient temperature. Certain indices might seem to indicate that the burn-out gradient of the polar materials between 15 K. and 60 K. might be several times greater than those measured on nobler materials at temperatures usual in the transformer art.

The mechanical resistance of insulators as also that of organic or metal structural materials used in the construction of transformers is similarly considerably increased when temperatures are lowered to the order of 15 K. to 60 K.

It has thus been found advantageous in the present invention to be able to allow insulators to share the mechanical resistance of the coils during short circuiting.

It is also possible, at these temperatures, to use magnetic materials, the Curie point which is lower than ambient temperature, for example of the order of 50 K. to K. This has the considerable advantage that certain of these materials such as dysprosium and holmium have saturation inductance considerably greater than that of the materials which may be used at above 0 C. Inductances of approximately 40 K./gauss may thus be used in the magnetic circuit of the new transformer, the volume of which will be reduced by at least a half. The magnetic materials conventionally used have, however, the advantage of leaving the magnetic circuit at ambient temperature in order to dissipate its losses towards the atmosphere.

The transformer shown in FIGURES 2 and 3 comprises a magnetic circuit 1 upon which are arranged three tanks 2 having two concentric walls 3 and 4 enclosing the coils adapted to be connected to the mains by conductors 5. A vacuum pump 6 is operative to produce a vacuum of the order of l0 to 10 torr between the two walls 3 and 4 of the tanks 2, in order to create suitable heat insulation conditions.

A pump 7 is operative to effect circulation of liquid hydrogen or neon in the coils. This neon or hydrogen is returned at the output of the coil by a reducing valve 8, and returns in a closed circuit to a liquifier 9. The pumps -6 and 7, the valve 8 and the liquifier 9 are all of conventional construction and are well known in the art.

Another solution according to the present invention consists in ensuring in the coils a circulation of gaseous hydrogen or neon, which may be under pressure greater than 1 atmosphere, and maintained at low temperature by any suitable means (heat exchange, reduction, evaporation, etc.)

The annular double walls 3 and 4 are constituted by an insulating material, such as a polyester or an epoXy resin reinforced with glass fibres.

The space between the two walls 3 and 4 contains a suitable heat insulator constituted for example as shown in FIGURE 4 by thin leaves of polyethylene terephthalate 11 covered with a layer of aluminum serving as a screen for heat radiations and by insulating packing 12 of expanded material, such as vulcanised sponge serving to resist mechanical stress.

Returning to FIGURE 3, it can be seen that an outlet 13 serves to connect the space 10 with the vacuum pump 6.

Within the tank are located two coils 14 and 15 (and additional coils if desired). These coils are constituted by a coil of thin sheets of aluminum 16 and 17 insulated either by anodised layers or by thin sheets 18 and 19 of insulating paper or Synthetic products, or if desired by a combination of said materials. Conducts 20 are provided within the coil for the circulation of the liquified gas between the input and output of the tank, shown respectively at 21 and 22. Insulation between the coils 14 and 15 is ensured by insulating screens 23. The coils 14 and 15 and the screens 23 are held in the tank by means of insulating packing 24.

This arrangement of the coils and the constitution of the conductors are by no means limiting and it will in certain cases be found more satisfactory to use imbricating, concentric or alternating coils or again rectangular conductors constituted by thin strips of aluminum with anodising insulation, regularly transposed in order to avoid any circulating currents, or again stranded or braided cables made from very fine wires, which may be insulated from each other by anodisation. Within the scope of the present invention it is also possible to use conductors of copper, tin or iron or any other metal for which the losses due to the Joules effect would be sufficiently low at a low temperature.

The insulators used for the leaves 18 and 19, screens 23, packing 24, as well as those suitable for impregnation or coating, are preferably chosen from among the materials whose dielectric strength between 15 K. and 60 K. is considerably greater than that measured on the same materials at ambient temperature such as polar materials such as polyethylene terephthalate, methyl polymethacrylate, and certain oxygenised or halogenated thermoplastic materials such as polyvinyl super chloride polyvinylidene chloride and trichlorofluorethylene.

The insulators share the mechanical resistance of the coils due to a suitable arrangement of said insulators such as the actual coiling of the leaves 18 and 19 and/or due to impregnation and/or coating of the coil by a synthetic product.

It has been found that transformers according to the present invention, as compared with conventional transformers, allow for a considerable decrease in losses, taking into account the power required by the liquified gas coolants and at the same time, an appreciable decrease in size.

FIGURE 5 shows an embodiment of the invention applied to the case of a shell-type transformer. All the elements shown in FIGURE 2 are also present in this case, and like references refer to like parts. The tanks 2 are also traversed by hydrogen or neon circulated by the pump 7 and provided with a double wall, within which vacuum is produced by means of the pump 6.

FIGURE 6 shows an embodiment of the invention using the magnetic properties of a material, whose Curie point is lower than ambient temperature. All the elements of FIGURE 2 are used, except that the three tanks 2 are replaced by a single tank 25 which encloses both the coils 14 and 15 shown here in a diagrammatic manner,

but in fact similar to those described in the preceding example and shown in FIGURE 3, and a magnetic circuit 1 which is constituted by stacks of thin sheets or wires of dysprosium or holmiurn or any other material of this type, the Curie point of which is lower than ambient temperature.

We claim:

1. An electrical transformer including a tank containing conductors immersed in a dielectric fluid, wherein said fluid is selected from the group consisting of hydrogen and neon at a temperature between 15 K. and 60 K. and wherein said conductors are of very pure metal.

2. An electrical transformer as set forth in claim 1, wherein said very pure metal conductors are selected from the group consisting of copper, aluminum, iron, tin, gold and indium and wherein the impurity rate of said metals is lower than 500 parts per million,

3. An electrical transformer comprising a magnetic circuit, a tank, coils dispersed in magneto inductive relationship with said magnetic circuit and located within said tank, means for circulating a dielectric fluid through said tank at a temperature between 15 K. and 60 K., said dielectric fluid being selected from the group consistingof hydrogen and neon, and said coils being wound from a metal whose impurity rate is less than 500 parts per million.

4. An electrical transformer comprising a tank, means for circulating a dielectric fluid through said tank at a temperature between 15 K. and 60 K., a magnetic circuit disposed outside said tank, coil windngs located on said magnetic circuit and within said tank, said dielectric fluid being selected from the group consisting of hydrogen and neon, and said coils being wound from a metal whose impurity rate is less than 500* parts per million.

5. An electrical transformer as set forth in claim 4, wherein said tank comprises a double wall of insulating material, and wherein said tank also houses a heat insulator under vacuum, said heat insulator comprising sheets of insulating material and mechanical packing of expanded material.

6. An electric transformer as set forth in claim 4, wherein said coils consist of wire whose turns are insulated from each other by a material selected from the group consisting of polyethylene terephthalate, methyl polymethacrylate, polyvinyl chloride, polyvinyl superchlm ride, polyvinylidene chloride, and trichlorofluorethylene.

7. An electric transformer comprising a tank, means for circulating a dielectric fluid through said tank, at a temperature between 15 K. and 60 K., a magnetic circuit outside said tank, coil windings located on said magnetic circuit and within said tank, said dielectric fluid being selected from the group hydrogen and neon, and said coils being wound from a metal whose impurity rate is less than 500 parts per million, said magnetic circuit being constituted of a material whose Curie point is lower than the ambient temperature.

8. An electrical transformer comprising a tank, means for circulating a dielectric fluid through said tank at a temperature between 15 K. and 60 K., a magnetic circuit disposed within said tank, coil windings located on said magnetic circuit and within said tank, said dielectric fluid being selected from the group hydrogen and neon, and said coils being wound from a metal whose impurity rate is less than 500 parts per mil-lion, said magnetic material being constituted of a material selected from the group consisting of dysprosium and holrmum.

9. In an electrical inductive apparatus having conductors surrounded by a dielectric fluid, said conductors comprising very pure metal maintained at low temperature, the improvement wherein said dielectric fluid is selected from the group consisting of hydrogen and neon maintained at a temperature between 15 K. and. 60 K.

(References on following page) 7 8 References Cited 3,201,728 8/1965 McWhirter 17915 X UNITED STATES 3,216, 84 GlfiOId 650,987 6/1900 Ostergren. 1 2,195,233 3/1940 Boyer n 336-205 LARAMIE ASKIN, Pllmary Exammer- 2,426,413 8/1947 Pollett. T. I. KOZMA, Assistant Examiner.

3,173,079 3/1965 McFee. 

